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United States Patent |
6,113,811
|
Kausch
,   et al.
|
September 5, 2000
|
Dichroic polarizing film and optical polarizer containing the film
Abstract
A dichroic polarizing film is made, for example, by, first combining
polyvinyl alcohol and a second polymer, such as, polyvinyl pyrrolidone or
a sulfonated polyester, in a solvent. The ratio of polyvinyl alcohol to
second polymer is between about 5:1 to 100:1 by weight. The film is coated
on a substrate, dried, and then stretched to orient at least a portion of
the film. The film incorporates a dichroic dye material, such as iodine,
to form a dichroic polarizer. This polarizer may be used in conjunction
with a multilayer optical film, such as a reflective polarizer, to form an
optical polarizer. The multilayer optical film may contain two or more
sets of polyester films, where at least one of the sets is birefringent
and orientable by stretching. The polyvinyl alcohol/second polymer film
and the multilayer optical film may be simultaneously stretched to orient
both polymer films.
Inventors:
|
Kausch; William L. (Cottage Grove, MN);
Williams; Brian H. (White Bear Lake, MN);
Merrill; William W. (White Bear Lake, MN)
|
Assignee:
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3M Innovative Properties Company (St. Paul, MN)
|
Appl. No.:
|
006458 |
Filed:
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January 13, 1998 |
Current U.S. Class: |
252/585; 428/1.31; 428/221; 524/503; 524/603; 525/58 |
Intern'l Class: |
G02C 007/12; G02B 005/30; C08L 029/04; C09K 019/52 |
Field of Search: |
524/503,603
525/58
428/221
252/585
|
References Cited
U.S. Patent Documents
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4304901 | Dec., 1981 | O'Neill et al. | 524/603.
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4446305 | May., 1984 | Rogers et al. | 528/348.
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4520189 | May., 1985 | Rogers et al. | 528/331.
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4521588 | Jun., 1985 | Rogers et al. | 528/363.
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4544693 | Oct., 1985 | Surgant | 524/503.
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4659563 | Apr., 1987 | Dobkin | 424/86.
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4720426 | Jan., 1988 | Englert et al. | 428/344.
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4890365 | Jan., 1990 | Langer | 26/73.
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5122557 | Jun., 1992 | Claussen et al. | 524/503.
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5188760 | Feb., 1993 | Hikmet et al. | 252/299.
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5211878 | May., 1993 | Reiffenrath et al. | 252/299.
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5269995 | Dec., 1993 | Ramanathan et al. | 264/171.
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5294657 | Mar., 1994 | Melendy et al. | 524/270.
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5316703 | May., 1994 | Schrenk | 264/1.
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5319478 | Jun., 1994 | Fijnfschilling et al. | 359/53.
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5340504 | Aug., 1994 | Claussen | 252/585.
|
5389324 | Feb., 1995 | Lewis et al. | 264/171.
|
5427835 | Jun., 1995 | Morrison et al. | 428/96.
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5448404 | Sep., 1995 | Schrenk et al. | 359/584.
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5486935 | Jan., 1996 | Kalmanash | 359/37.
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5486949 | Jan., 1996 | Schrenk et al. | 359/498.
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5612820 | Mar., 1997 | Schrenk et al. | 359/498.
|
5629055 | May., 1997 | Revol et al. | 428/1.
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5686979 | Nov., 1997 | Weber et al. | 349/96.
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5699188 | Dec., 1997 | Gilbert et al. | 359/584.
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5721603 | Feb., 1998 | De Vaan et al. | 349/194.
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5744534 | Apr., 1998 | Ishiharada et al. | 524/442.
|
5751388 | May., 1998 | Larson | 349/96.
|
5767935 | Jun., 1998 | Ueda et al. | 349/112.
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5770306 | Jun., 1998 | Suzuki et al. | 428/328.
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5783120 | Jul., 1998 | Ouderkirk et al. | 264/134.
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5793456 | Aug., 1998 | Broer et al. | 349/98.
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5808794 | Sep., 1998 | Weber et al. | 359/487.
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|
5825543 | Oct., 1998 | Ouderkirk et al. | 359/494.
|
Foreign Patent Documents |
0564925 | Aug., 1993 | EP | .
|
4406426 | Aug., 1995 | DE | .
|
1084820 | Sep., 1964 | GB | .
|
WO 95/27919 | Apr., 1995 | WO | .
|
WO 95/17303 | Jun., 1995 | WO | .
|
WO 95/17692 | Jun., 1995 | WO | .
|
WO 95/17691 | Jun., 1995 | WO | .
|
WO 95/17699 | Jun., 1995 | WO | .
|
WO 96/19347 | Jun., 1996 | WO | .
|
WO 97/01788 | Jan., 1997 | WO.
| |
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|
WO 97/01440 | Jan., 1997 | WO | .
|
WO 97/32226 | Sep., 1997 | WO | .
|
Other References
Schrenk et al., Nanolayer polymeric optical films, Tappi Journal, pp.
169-174, Jun., 1992.
|
Primary Examiner: Yoon; Tae
Attorney, Agent or Firm: Burtis; John A.
Claims
We claim:
1. A polarizing film, comprising:
a polymeric film comprising a mixture of polyvinyl alcohol and a
water-soluble or water-dispersible second polymer, the polymer film being
oriented; and
dichroic dye material incorporated in the polymeric film.
2. The polarizing film of claim 1, wherein the polymeric film is formed by
removal of a solvent from a dispersion of polyvinyl alcohol and the second
polymer in the solvent.
3. The polarizing film of claim 2, wherein the solvent is water.
4. The polarizing film of claim 1, wherein the second polymer comprises a
water-soluble polymer.
5. The polarizing film of claim 1, wherein the second polymer comprises a
polar polymer.
6. The polarizing film of claim 1, wherein the second polymer comprises a
polyester.
7. The polarizing film of claim 6, wherein the second polymer comprises a
sulfonated polyester.
8. The polarizing film of claim 1, wherein the second polymer comprises
polyvinyl pyrrolidone.
9. The polarizing film of claim 1, wherein the ratio of polyvinyl alcohol
to the second polymer is between 8:1 to 20:1 by weight.
10. The polarizing film of claim 1, wherein the dichroic dye material
stains the polymeric film.
11. The polarizing film of claim 2, wherein the dichroic dye material is
included in the dispersion of the polyvinyl alcohol and the second
polymer.
12. The polarizing film of claim 1, wherein the polymeric film is oriented
by stretching the film in at least one direction.
13. A polarizing film, comprising:
a polymeric film comprising a mixture of polyvinyl alcohol and a
water-soluble or water-dispersible second polymer, wherein the film is
oriented by stretching along a film dimension, the second polymer being
provided in an amount sufficient to reduce significantly cracking of the
polymeric film during stretching; and
dichroic dye material incorporated in the polymer film.
14. The polarizing film of claim 13, wherein the second polymer is provided
in an amount sufficient to prevent cracking of the polymeric film during
stretching.
15. The polarizing film of claim 13, wherein the film is formed by removal
of a solvent from a dispersion of polyvinyl alcohol and the second polymer
in the solvent.
16. The polarizing film of claim 15, wherein the solvent is water.
17. The polarizing film of claim 13, wherein the film is oriented by
stretching along a film dimension so that the length of the film in that
film dimension is at least six times the length of the film prior to
stretching.
18. The polarizing film of claim 13, wherein the second polymer comprises a
polar polymer.
19. The polarizing film of claim 13, wherein the second polymer comprises a
polyester.
20. The polarizing film of claim 19, wherein the second polymer comprises a
sulfonated polyester.
21. The polarizing film of claim 13, wherein the second polymer comprises
polyvinyl pyrrolidone.
Description
FIELD OF THE INVENTION
The present invention relates to a polarizing film and an optical polarizer
containing the film. More particularly, the invention relates to a
dichroic polarizing film made from a dispersion or solution of a polyvinyl
alcohol and a second polymer, and an optical polarizer containing the
film.
BACKGROUND OF THE INVENTION
Optical polarizing film is widely used for glare reduction and for
increasing optical contrast in such products as sunglasses and Liquid
Crystal Displays (LCD). One of the most commonly used types of polarizers
for these applications is a dichroic polarizer which absorbs light of one
polarization and transmits light of the other polarization. One type of
dichroic polarizer is made by incorporating a dye into a polymer matrix
which is stretched in at least one direction. Dichroic polarizers may also
be made by uniaxially stretching a polymer matrix and staining the matrix
with a dichroic dye. Alternatively, a polymer matrix may be stained with
an oriented dichroic dye. Dichroic dyes include anthraquinone and azo
dyes, as well as iodine. Many commercial dichroic polarizers use polyvinyl
alcohol as the polymer matrix for the dye.
Another type of polarizer is a reflective polarizer which reflects light of
one polarization and transmits light of another orthogonal polarization.
One type of reflective polarizer is made by forming a stack of alternating
sets of polymer layers, one of the sets being birefringent to form
reflective interfaces in the stack. Typically, the indices of refraction
of the layers in the two sets are approximately equal in one direction so
that light polarized in a plane parallel to that direction is transmitted.
The indices of refraction are typically different in a second, orthogonal
direction so that light polarized in a plane parallel to the orthogonal
direction is reflected.
One measure of performance for polarizers is the extinction ratio. The
extinction ratio is the ratio of a) light transmitted by the polarizer in
a preferentially transmitted polarization state to b) light transmitted in
an orthogonal polarization state. These two orthogonal states are often
related to the two linear polarizations of light. However, other types of
orthogonal states, such as, left and right-handed circular polarizations
or two orthogonal elliptical polarizations may also be used. The
extinction ratios of dichroic polarizers vary over a wide range depending
on their specific construction and target application. For example,
dichroic polarizers may have extinction ratios between 5:1 and 3000:1.
Dichroic polarizers used in display systems typically have extinction
ratios which are preferably greater than 100:1 and even more preferably
greater than 500:1.
Dichroic polarizers typically absorb light in the non-transmission
polarization. However, dichroic polarizers also absorb some of the light
having the high transmission polarization. The amount of this absorption
depends on the details of the construction of the polarizer and the
designed extinction ratio. For high performance display polarizers, such
as those used in LCDs, this absorption loss is typically between about 5
and 15%. The reflectivity of these polarizers for light having the
absorption (i.e., low transmission) polarization tends to be small. Even
with surface reflections included, this reflectivity is typically less
than 10% and usually less than 5%.
Reflective polarizers typically reflect light having one polarization and
transmit light having an orthogonal polarization. Reflective polarizers
often have incomplete reflectivity of the high extinction polarization
over a wavelength region of interest. Typically, the reflectivity is
greater than 50% and is often greater than 90% or 95%. A reflective
polarizer will also typically have some absorption of light having the
high transmission polarization. Typically, this absorption is less than
about 5 to 15%.
SUMMARY OF THE INVENTION
The above two types of polarizers may be combined to make a single optical
polarizer, thereby incorporating the useful characteristics of both types
of polarizers. These polarizers may be formed and, optionally, oriented
together. Unfortunately, the polyvinyl alcohol film used in many dichroic
polarizers tends to crack under the processing conditions necessary to
prepare some reflective polarizers, including, for example, those which
use polyethylene naphthalate (PEN) or coPEN optical layers. These
reflective polarizers may be formed by stretching a polymeric film at
processing temperatures, such as 135 to 180.degree. C., and a stretch
ratio of between 2:1 and 10:1. There is a need for a dichroic film layer
that does not crack under these processing conditions.
Dichroic polarizers may also be used with other optical devices, such as
other types of reflective polarizers and mirrors. The combination of a
dichroic polarizer with an IR mirror may be useful for reducing glare. The
formation of the dichroic polarizer in combination with the mirror retains
the processing difficulties mentioned above, especially when the mirror is
made using oriented polyester layers.
Accordingly, the present invention relates to dichroic polarizing films and
their use in optical polarizers. In one embodiment, a polarizing film
includes a polymeric film which contains polyvinyl alcohol and a second
polymer. The polymeric film is oriented and incorporates a dichroic dye
material. The dichroic dye material may be incorporated before or after
stretching of the film.
Another embodiment is an optical device which includes a substrate and a
polarizing film. The polarizing film is disposed on the substrate and
contains polyvinyl alcohol and a second polymer. The polymeric film is
oriented and incorporates a dichroic dye material.
A further embodiment is a method of making an optical device. The method
includes forming a dispersion of polyvinyl alcohol and a second polymer in
a solvent. A substrate is coated with the dispersion/solution and then the
solvent is removed from the dispersion to form a polymeric film. The
polymeric film is then oriented by stretching in at least one direction. A
dichroic dye material is also incorporated in the polymeric film.
Another embodiment is a display device made from a polarizing film. The
polarizing film includes a polymeric film which contains polyvinyl alcohol
and a second polymer. The polymeric film is oriented and incorporates a
dichroic dye material.
The above summary of the invention is not intended to describe each
illustrated embodiment or every implementation of the present invention.
The figures and the detailed description which follow more particularly
exemplify these embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may be more completely understood in consideration of the
following detailed description of various embodiments of the invention in
connection with the accompanying drawings, in which:
FIG. 1 is a side elevational view of one embodiment of an optical polarizer
according to the present invention;
FIG. 2 is a side elevational view of one embodiment of a multilayer optical
film for use in the optical polarizer of FIG. 1; and
FIG. 3 is a side elevational view of another embodiment of a multilayer
optical film for use in the optical polarizer of FIG. 1.
While the invention is amenable to various modifications and alternative
forms, specifics thereof have been shown by way of example in the drawings
and will be described in detail. It should be understood, however, that
the intention is not to limit the invention to the particular embodiments
described. On the contrary, the intention is to cover all modifications,
equivalents, and alternatives falling within the spirit and scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION
The invention is directed to optical polarizers and in particular to
dichroic polarizers. The invention is also directed to the formation of
these polarizers and to their use with other optical elements, such as
reflective polarizers, mirrors, and IR mirrors.
Some conventional dichroic polarizers 11 are made using polyvinyl alcohol
films. These films are well-known in the art and have been used, upon
incorporation of a dye material, as dichroic polarizers. To function as a
dichroic polarizer, the polyvinyl alcohol film is typically stretched to
orient the film. When stained, the orientation of the film determines the
optical properties (e.g., the axis of extinction) of the film. One use of
these films is in conjunction with multilayer optical films which are also
stretched to orient one or more layers of the film to form, for example,
reflective polarizers and mirrors. Examples of such multilayer optical
films may be found in U.S. patent application Ser. No. 08/402,041, U.S.
patent application Ser. No. 09/006,601 entitled "Modified Copolyesters and
Improved Multilayer Reflective Films", U.S. patent application Ser. No.
09/006,288 entitled "Process for Making Multilayer Optical Film" and U.S.
patent application Ser. No. 09/006,455 entitled "Optical Film and Process
for the Manufacture Thereof" all of which are incorporated herein by
reference. Other multilayer optical films, reflective polarizers, mirrors,
and optical devices may also be used in conjunction with the dichroic
polarizers.
Unfortunately, polyvinyl alcohol films tend to crack under the stretching
conditions used in the formation of many reflective polarizers, including,
for example, those made from multilayer polyester films, and in
particular, polyester films containing naphthalate subunits such as PEN.
Although no particular theory is necessary to the invention, it is thought
that polyvinyl alcohol forms a hydrogen-bonded network which is incapable
of stretching under these conditions while maintaining its structural
integrity. The hydrogen-bonded network is strained and, finally, at one or
more points slips, thereby causing cracks. Experimentation indicates that
small molecule plasticizers do not solve this problem.
It has been found that the addition of a second polymer dispersible or
soluble in a solvent used in the formation of the polyvinyl alcohol film
significantly reduces cracking. The second polymer is included as either a
dispersion or a solution, depending on the nature of the second polymer,
and the terms "dispersion" and "solution" will be used interchangeably
herein. The second polymer is preferably water-soluble as water is a
common solvent for polyvinyl alcohol. More preferably, the second polymer
is a polar polymer. Suitable second polymers include, for example,
polyvinyl pyrrolidone and polyesters dispersible in the solvent of the
polyvinyl alcohol. Examples of water-soluble or water dispersible
polyesters include sulfonated polyesters, such as those describe in U.S.
Pat. No. 5,427,835, incorporated herein by reference. Suitable co-solvents
include, for example, polar solvents such as C1-C4 alcohols.
Typically, the polyvinyl alcohol and second polymer are mixed in a ratio of
between 5:1 and 100:1 by weight, and preferably between 8:1 and 20:1 by
weight. The solution is typically 1 to 50 wt. % solids, and preferably 5
to 25 wt. % solids. Although no particular theory is necessary to the
invention, it is thought that the addition of the second polymer separates
the hydrogen-bonded network into a large number of domains which may move
relative to each other when strained, thereby relieving the strain and
reducing the amount of cracking.
The polyvinyl alcohol film may be made by a variety of techniques. One
exemplary method for making the film includes combining the polyvinyl
alcohol and the second polymer in a solvent according to the
above-mentioned ratios and weight percentages. This dispersion/solution of
the two polymers is then applied to the surface of a substrate. The
substrate may be another film, a multilayer stack, a plastic object, or
any other surface which allows stretching of the polyvinyl alcohol film.
Application of the dispersion/ solution may be accomplished by a variety
of known methods, including, for example, coating the substrate using
techniques, such as shoe coating, extrusion coating, roll coating, curtain
coating, or any other coating method capable of providing a uniform
coating. The substrate may be coated with a primer or treated with a
corona discharge to help anchor the polyvinyl alcohol film to the
substrate. Typically, the thickness of the coating is 25 to 500 .mu.m when
wet and preferably 50 to 125 .mu.m. After coating, the polyvinyl alcohol
film is dried at a temperature typically between 100.degree. C. and
150.degree. C. The film is then stretched using, for example, length
orienters or tenter clips to orient the film. In some embodiments, the
film is removed from the substrate. The film may then be adhered to
another surface, if desired. The polyvinyl alcohol film, when stained, can
then be used as a dichroic polarizer. However, it will be understood that
other uses may be made of the polyvinyl alcohol film.
A finished polyvinyl alcohol film typically includes a dichroic dye
material to form a dichroic polarizer. The dichroic dye material may
include dyes, pigments, and the like. Suitable dye materials for use in
the dichroic polarizer film include, for example, iodine, as well as
anthraquinone and azo dyes, such as Congo Red (sodium
diphenyl-bis-.alpha.-naphthylamine sulfonate), methylene blue, stilbene
dye (Color Index (CI)=620), and 1,1'-diethyl-2,2"-cyanine chloride (CI=374
(orange) or CI=518 (blue)). The properties of these dyes, and methods of
making them, are described in E. H. Land, Colloid Chemistry (1946). Still
other dichroic dyes, and methods of making them, are discussed in the Kirk
Othmer Encyclopedia of Chemical Technology, Vol. 8, pp. 652-661 (4th Ed.
1993), and in the references cited therein.
The dichroic dye material may be added to the dispersion or solution of the
polyvinyl alcohol and second polymer prior to coating. Alternatively, a
polyvinyl alcohol film may be stained with a staining composition, such
as, for example, an iodine-containing solution. The staining of the
polyvinyl alcohol film may occur before or after the film is drawn. In
some cases, the dichroic dye material may not be able to withstand the
drawing conditions and should therefore be applied to the polyvinyl
alcohol film after drawing.
One example of a suitable staining composition is an iodine-containing
solution. The iodine stained film may be stabilized using, for example, a
boron-containing composition, such as a boric acid/borax solution. Other
stains may require different stabilizers. The concentrations of the
staining or stabilizing compositions, as well as the temperature at which
the staining or stabilization occurs and the time of contact with each
solution, may vary widely without compromising the stain.
Various other components may be added to the dispersion/solution of
polyvinyl alcohol and the second polymer. For example, a surfactant may be
added to facilitate wetting of the substrate. A wide variety of
surfactants may be used, including, for example, Triton X-100 (Union
Carbide Chemicals and Plastics Company, Inc., Danbury, Conn.). The
surfactant is typically about 1% or less of the solution, and preferably
about 0.5% or less. The surfactant is preferably nonionic so that it does
not interfere with polar groups on the polymer.
Another optional additive is a drying aid which facilitates film formation
on drying. Example of a suitable drying aids includes N-methyl-pyrrolidone
and butyl carbitol. The drying aid is typically about 10% or less of the
solution, and preferably about 5% or less.
Various functional layers or coatings may be added to the optical films and
devices of the present invention to alter or improve their physical or
chemical properties, particularly along the surface of the film or device.
Such layers or coatings may include, for example, slip agents, low
adhesion backside materials, conductive layers, antistatic coatings or
films, barrier layers, flame retardants, UV stabilizers, abrasion
resistant materials, optical coatings, compensation films, retardation
films, diffuse adhesives, and/or substrates designed to improve the
mechanical integrity or strength of the film or device. In addition, an
adhesive may be applied to the polyvinyl alcohol film to adhere the film
to the substrate. This may be particularly useful when the polyvinyl
alcohol film is removed from a first substrate and then placed on a second
substrate. A variety of adhesives may be used including, for example,
resins and pressure sensitive adhesives (PSA). When choosing a suitable
adhesive, the optical properties of the adhesive are usually considered.
The addition of a second polymer to the polyvinyl alcohol film provides an
improved dichroic polarizer which is compatible with the simultaneous
orientation of the polyvinyl alcohol film and a multilayer optical film,
such as a reflective polarizer or mirror film. The advantage of using the
improved dichroic polarizer is that the dichroic and multilayer optical
film may be oriented together, thereby forming, for example, an optical
polarizer which may have dichroic and reflective elements that are more
perfectly aligned. Furthermore, the addition of a second polymer to the
polyvinyl alcohol film often improves the adhesion of the film to a
substrate.
An exemplary process for forming optical devices includes, first, forming a
multilayer optical film, as described below. This multilayer optical film
is coated or laminated with a polyvinyl alcohol film which incorporates
the second polymer. This may be accomplished using well-known devices,
such as, for example, shoe coating, extrusion coating, roll coating,
curtain coating, or any other coating method capable of providing a
uniform coating.
The multilayer optical film and the polyvinyl alcohol film are then
simultaneously drawn to form an oriented multilayer optical film and an
oriented polyvinyl alcohol film. In some embodiments, the multilayer
optical film is drawn multiple times. In these embodiments, the polyvinyl
alcohol film is often coated or laminated on the multilayer optical film
prior to the final draw. In alternative embodiments, the two films may be
drawn and oriented separately. Known devices may be used to draw the two
films, including, for example, tenters or long orienters. Drawing the
polyvinyl alcohol film and the multilayer optical film together typically
results in the orientation axis of the polyvinyl alcohol layer being
coincident with the axis of final orientation of the multilayer optical
film, which may be either a polarizer film or a mirror film. Dichroic dye
material may be added prior to drawing the films, or may be incorporated
later by, for example, staining the polyvinyl alcohol film, as described
above.
A number of different combinations of dichroic polarizer and multilayer
polymer films may be formed. For example, a visible band dichroic and
reflective polarizer combination, an IR band mirror and dichroic polarizer
combination, an IR band polarizer and dichroic polarizer combination,
among others, may be formed.
FIG. 1 illustrates an exemplary device, namely an optical polarizer 10
which includes a dichroic polarizer 11 and a reflective polarizer 12. This
combination of two different types of polarizers may create an optical
polarizer with a high reflection/absorption of light of one polarization
and a high transmission of light with a second, orthogonal polarization.
Typically, the two polarizers are aligned with respect to each other to
provide maximum transmissivity of light having a particular polarization.
The dichroic polarizer 11 is typically in close proximity to the reflective
polarizer 12, although this is not necessary. Preferably, the two
polarizers 11, 12 are bonded to each other to eliminate any air gap.
The reflective polarizer 12 usually reflects a substantial portion of light
having a first polarization and transmits most of the light having a
second, orthogonal polarization. The dichroic polarizer 11 typically
absorbs most of light having a third polarization and transmits a
substantial portion of light having a fourth, orthogonal polarization.
Often, the optical polarizer 10 is formed by orienting the reflective
polarizer 12 and the dichroic polarizer 11 so that they transmit light of
a particular polarization (i.e., the second and fourth polarization are
the same) and reflect/absorb light of an orthogonal polarization (i.e.,
the first and third polarizations are the same). The present invention
will be discussed with reference to this particular configuration.
However, other configurations in which the reflective polarizer 12 and the
dichroic polarizer 11 are oriented in a different manner with respect to
each other are also possible and included within the invention.
In use, the combined polarizers are illuminated on one or both of the
outside facing surfaces, as illustrated in FIG. 1. Ray 13 is light having
a polarization that is preferentially reflected by the reflective
polarizer 12 to form ray 14. Ray 15 is light from ray 13 that is
transmitted by the reflective polarizer 12. Typically, ray 15 is much less
intense than ray 14. In addition, ray 15 is usually attenuated by the
dichroic polarizer 11. Light ray 16, which is orthogonally polarized to
ray 13, is preferentially transmitted by the reflective polarizer 12 and
is typically only slightly attenuated by the dichroic polarizer 11.
Ray 17 is light having a polarization that is preferentially absorbed by
the dichroic polarizer 11, and which preferably has the same polarization
as ray 13. The portion of ray 17 which is transmitted by the dichroic
polarizer 11 is further attenuated by reflection off the reflective
polarizer 12, thereby forming ray 18. Light ray 19 is polarized
orthogonally to ray 17 and preferably has the same polarization as ray 16.
Ray 19 is preferentially transmitted by both the dichroic polarizer 11 and
the reflective polarizer 12.
Combining the dichroic polarizer 11 with the reflective polarizer 12
results in an optical polarizer 10 which has a higher extinction ratio of
the transmitted light than would be the case with a dichroic polarizer by
itself. This allows for the use of a dichroic polarizer with a lower
extinction ratio. This may be useful, as dichroic polarizers typically
absorb some of the light that is to be transmitted. Using a dichroic
polarizer with a lower extinction ratio may increase the amount of light
of the desired polarization which is transmitted. For light polarized
along the extinction axis, the preferred extinction percentage for the
dichroic polarizer is 10% or greater, more preferred is 55% or greater,
and most preferred is 70% or greater. The best choice of dichroic and
reflective polarizers depends on the design goals, including the allowed
reflectivity from the dichroic polarizer side of the film, the extinction
ratio of the reflective polarizer, and the desired final polarizer
contrast.
The combination of the reflective polarizer with the dichroic polarizer has
other advantages. For example, this combination has a high reflectivity
from one side of the film for one polarization and a low reflectivity from
the other side of the film. The combination of these two characteristics
may be useful in a number of systems including direct view LCDs. For
example, a direct view LCD might have a back side reflectivity of 1% and
require a final extinction ratio greater than 1000:1. To achieve 1%
reflectivity when combined with a reflective polarizer with an
approximately 100% reflectivity, the dichroic polarizer would need to
transmit 10% or less of light polarized in the extinction direction. If
the reflective polarizer has an extinction ratio of 50:1, then the
dichroic polarizer would typically require an extinction ratio of at least
20:1 to achieve the final extinction ratio of 1000:1.
The reflective polarizer 12 may contain internal structure, such as
interfaces between different materials, where the index is not exactly
matched in the appropriate directions, or other scattering centers. Both
of these types of internal structure may interfere with light which would
normally be transmitted through the polarizer. In general, it is preferred
that the reflection of light having the transmission polarization by the
reflective polarizer 12 be about 30% or less, more preferably about 20% or
less, and most preferably about 15% or less. In addition, the reflectivity
of the reflective polarizer depends on the wavelength range and the angle
of incident light. The preferred reflection percentage by the reflective
polarizer 12 for light having the reflection polarization and within a
wavelength range of interest is 20% or greater, more preferably 50% or
greater and most preferably 90% or greater.
Similar design features and parameters may be used when the multilayer
optical film is a mirror or IR mirror. The preferred reflection percentage
by a mirror for light with a wavelength range of interest, whether visible
or infrared, is 20% or greater, more preferably about 50% or greater, and
most preferably about 90% or greater.
One example of a useful multilayer optical film 20 is shown in FIG. 2. This
multilayer optical film 20 may be used to make reflective polarizers,
mirrors and other optical devices. The multilayer optical film 20 includes
one or more first optical layers 22, one or more second optical layers 24,
and one or more non-optical layers 28. The first optical layers 22 are
often birefringent polymer layers which are uniaxially- or
biaxially-oriented. In some embodiments, the first optical layers 22 are
not birefringent. The second optical layers 24 may be polymer layers which
are birefringent and uniaxially- or biaxially-oriented. More typically,
however, the second optical layers 24 have an isotropic index of
refraction which is different than at least one of the indices of
refraction of the first optical layers 22 after orientation. The methods
of manufacture and use, as well as design considerations for the
multilayer optical films 20 are described in detail in U.S. patent
application Ser. No. 08/402,041 entitled "Multilayered Optical Film", U.S.
patent application Ser. No. 09/006,601 entitled "Modified Copolyesters and
Improved Multilayer Reflective Films", U.S. patent application Ser. No.
09/006,288 entitled "Process for Making Multilayer Optical Film", all of
which are herein incorporated by reference. Although, the present
invention will be primarily exemplified by multilayer optical films 20
with second optical layers 24 which have an isotropic index of refraction,
the principles and examples described herein may be applied to multilayer
optical films 20 with second optical layers 24 that are birefringent, as
described in, for example, U.S. patent application Ser. No. 09/006,455
entitled "Optical Film and Process for the Manufacture Thereof" which is
also herein incorporated by reference.
Additional sets of optical layers, similar to the first and second optical
layers 22, 24, may also be used in the multilayer optical film 20. The
design principles disclosed herein for the sets of first and second
optical layers may be applied to any additional sets of optical layers.
Furthermore, it will be appreciated that, although only a single stack 26
is illustrated in FIG. 2, the multilayer optical film 20 may be made from
multiple stacks that are subsequently combined to form the film 20.
The optical layers 22, 24 and, optionally, one or more of the non-optical
layers 28 are typically placed one on top of the other to form a stack 26
of layers. Usually the optical layers 22, 24 are arranged as alternating
pairs, as shown in FIG. 2, to form a series of interfaces between layers
with different optical properties. The optical layers 22, 24 are typically
less than 1 .mu.m thick, although thicker layers may be used. Furthermore,
although FIG. 2 shows only six optical layers 22, 24, many multilayer
optical films 20 have a large number of optical layers. Typical multilayer
optical films 20 have about 2 to 5000 optical layers, preferably about 25
to 2000 optical layers, more preferably about 50 to 1500 optical layers,
and most preferably about 75 to 1000 optical layers.
The non-optical layers 28 are polymer films that are disposed within (see
FIG. 3) and/or over (see FIG. 2) the stack 26 to protect the optical
layers 22, 24 from damage, to aid in the co-extrusion processing, and/or
to enhance post-processing mechanical properties. The non-optical layers
28 are often thicker than the optical layers 22, 24. The thickness of the
non-optical layers 28 is usually at least two times, preferably at least
four times, and more preferably at least ten times, the thickness of the
individual optical layers 22, 24. The thickness of the non-optical layers
28 may be varied to obtain a particular thickness of the optical film 20.
Typically, one or more of the non-optical layers 28 are placed so that at
least a portion of the light to be transmitted, polarized, and/or
reflected by the optical layers 22, 24, also travels through the
non-optical layers (i.e., the non-optical layers are placed in the path of
light which travels through or is reflected by the optical layers 22, 24).
As a non-limiting example, the optical layers 22, 24 and the non-optical
layers 28 of the multilayer optical film 20 may be made using polymers,
such as polyesters. The term "polymer" includes polymers and copolymers,
as well as polymers and/or copolymers which may be formed in a miscible
blend, for example, by coextrusion or by reactions, including, for
example, transesterification. Polyesters have carboxylate and glycol
subunits which are generated by reactions of carboxylate monomer molecules
with glycol monomer molecules. Each carboxylate monomer molecule has two
or more carboxylic acid or ester functional groups and each glycol monomer
molecule has two or more hydroxy functional groups. The carboxylate
monomer molecules may all be the same or there may be two or more
different types of molecules. The same applies to the glycol monomer
molecules.
The properties of a polymer layer or film vary with the particular choice
of monomer molecules. One example of a polyester useful in multilayer
optical films is polyethylene naphthalate (PEN) which can be made, for
example, by reactions of naphthalene dicarboxylic acid with ethylene
glycol.
Suitable carboxylate monomer molecules for use in forming the carboxylate
subunits of the polyester layers include, for example, 2,6-naphthalene
dicarboxylic acid and isomers thereof, terephthalic acid; isophthalic
acid; phthalic acid; azelaic acid; adipic acid; sebacic acid; norbornene
dicarboxylic acid; bi-cyclooctane dicarboxylic acid; 1,6-cyclohexane
dicarboxylic acid and isomers thereof; t-butyl isophthalic acid,
tri-mellitic acid, sodium sulfonated isophthalic acid; 2,2'-biphenyl
dicarboxylic acid and isomers thereof; and lower alkyl esters of these
acids, such as methyl or ethyl esters. The term "lower alkyl" refers, in
this context, to C1-C10 straight-chained or branched alkyl groups. Also
included within the term "polyester" are polycarbonates which are derived
from the reaction of glycol monomer molecules with esters of carbonic
acid.
Suitable glycol monomer molecules for use in forming glycol subunits of the
polyester layers include ethylene glycol; propylene glycol; 1,4-butanediol
and isomers thereof; 1,6-hexanediol; neopentyl glycol; polyethylene
glycol; diethylene glycol; tricyclodecanediol; 1,4-cyclohexanedimethanol
and isomers thereof, norbornanediol; bicyclo-octanediol; trimethylol
propane; pentaerythritol; 1,4-benzenedimethanol and isomers thereof,
bisphenol A; 1,8-dihydroxy biphenyl and isomers thereof, and
1,3-bis(2-hydroxyethoxy)benzene.
Non-polyester polymers are also useful in creating polarizer or mirror
films. For example, layers made from a polyester such as polyethylene
naphthalate may be combined with layers made from an acrylic polymer to
form a highly reflective mirror film. In addition, polyether imides may
also be used with polyesters, such as PEN and coPEN, to generate a
multilayer optical film 20. Other polyester/non-polyester combinations,
such as polybutylene terephthalate and polyvinyl chloride, may also be
used.
Multilayered optical films may also be made using only non-polyesters. For
example, poly(methyl methacrylate) and polyvinylidene fluoride may be used
to make layers for a multilayer optical film 20. Another non-polyester
combination is atactic or syndiotactic polystyrene and polyphenylene
oxide. Other combinations may also be used.
The first optical layers 22 are typically orientable polymer films, such as
polyester films, which may be made birefringent by, for example,
stretching the first optical layers 22 in a desired direction or
directions. The term "birefringent" means that the indices of refraction
in orthogonal x, y, and z directions are not all the same. For films or
layers in a film, a convenient choice of x, y, and z axes is shown in FIG.
2 in which the x and y axes correspond to the length and width of the film
or layer and the z axis corresponds to the thickness of the layer or film.
In the embodiment illustrated in FIG. 2, the multilayer optical film 20
has several optical layers 22, 24 which are stacked one on top of the
other in the z-direction.
The first optical layers 22 may be uniaxially-oriented, for example, by
stretching in a single direction. A second orthogonal direction may be
allowed to neck into some value less than its original length. In one
embodiment, the direction of stretching substantially corresponds to
either the x or y axis shown in FIG. 2. However, other directions may be
chosen. A birefringent, uniaxially-oriented layer typically exhibits a
difference between the transmission and/or reflection of incident light
rays having a plane of polarization parallel to the oriented direction
(i.e., stretch direction) and light rays having a plane of polarization
parallel to a transverse direction (i.e., a direction orthogonal to the
stretch direction). For example, when an orientable polyester film is
stretched along the x axis, the typical result is that n.sub.x
.notident.n.sub.y, where n.sub.x and n.sub.y are the indices of refraction
for light polarized in a plane parallel to the "x" and "y" axes,
respectively. The degree of alteration in the index of refraction along
the stretch direction will depend on factors such as the amount of
stretching, the stretch rate, the temperature of the film during
stretching, the thickness of the film, the variation in film thickness,
and the composition of the film. Typically, the first optical layers 22
have an in-plane birefringence (the absolute value of n.sub.x -n.sub.y)
after orientation of 0.04 or greater at 632.8 nm, preferably about 0.1 or
greater, and more preferably about 0.2 or greater. All birefringence and
index of refraction values are reported for 632.8 nm light unless
otherwise indicated.
Polyethylene naphthalate (PEN) is an example of a useful material for
forming the first optical layers 22 because it is highly birefringent
after stretching. The refractive index of PEN for 632.8 nm light polarized
in a plane parallel to the stretch direction increases from about 1.62 to
as high as about 1.87. Within the visible spectrum, PEN exhibits a
birefringence of 0.20 to 0.40 over a wavelength range of 400-700 nm for a
typical high orientation stretch (e.g., a material stretched to five or
more times its original dimension at a temperature of 130.degree. C. and
an initial strain rate of 20%/min).
The birefringence of a material can be increased by increasing the
molecular orientation. Many birefringent materials are crystalline or
semicrystalline. The term "crystalline" will be used herein to refer to
both crystalline and semicrystalline materials. PEN and other crystalline
polyesters, such as polybutylene naphthalate (PBN), polyethylene
terephthalate (PET) and polybutylene terephthalate (PBT), are examples of
crystalline materials useful in the construction of birefringent film
layers such as is often the case for the first optical layers 22. In
addition, some copolymers of PEN, PBN, PET, and PBT are also crystalline
or semicrystalline. The addition of a comonomer to PEN, PBN, PET, or PBT
may enhance other properties of the material including, for example,
adhesion to the second optical layers 24 or the non-optical layers 28
and/or the lowering of the working temperature (i.e., the temperature for
extrusion and/or stretching the film).
If the polyester material of the first optical layers 22 contains more than
one type of carboxylate subunit, then the polyester may be a block
copolyester to enhance adhesion to other layers (e.g., the second optical
layers 24 or non-optical layers 28) made from block copolymers having
similar blocks. Random copolyesters may also be used.
Referring again to FIGS. 2 and 3, one or more of the non-optical layers 28
may be formed as a skin layer over at least one surface of stack 26 as
illustrated in FIG. 2, to, for example, protect the optical layers 22, 24
from physical damage during processing and/or afterwards. In addition, one
or more of non-optical layers 28 may be formed within the stack 26 of
layers, as illustrated in FIG. 3, to, for example, provide greater
mechanical strength to the stack or to protect the stack during
processing.
The non-optical layers 28 ideally do not significantly participate in the
determination of optical properties of the multilayer optical film 20, at
least across the wavelength region of interest. The non-optical layers 28
are typically not birefringent or orientable but in some cases this may
not be true. Typically, when the non-optical layers 28 are used as skin
layers there will be at least some surface reflection. If a multilayer
optical film 20 is to be a reflective polarizer, the non-optical layers
preferably have an index of refraction which is relatively low. This
decreases the amount of surface reflection. If the multilayer optical film
20 is to be a mirror, the non-optical layers 28 preferably have an index
of refraction which is high, to increase the reflection of light.
When the non-optical layers 28 are found within the stack 26, there will
typically be at least some polarization or reflection of light by the
non-optical layers 28 in combination with the optical layers 22, 24
adjacent to the non-optical layers 28. Typically, however, the non-optical
layers 28 have a thickness which dictates that light reflected by the
non-optical layers 28 within the stack 26 has a wavelength outside the
region of interest, for example, in the infrared region for visible light
polarizers or mirrors.
The non-optical layers 28 may also be made from copolyesters similar to the
second optical layers 24, using similar materials and similar amounts of
each material. In addition, other polymers may also be used, as described
above with respect to the second optical layers 24. It has been found that
the use of coPEN (i.e., a copolymer of PEN) or other copolymer material
for skin layers (as seen in FIG. 2) reduces the splittiness (i.e., the
breaking apart of a film due to strain-induced crystallinity and alignment
of a majority of the polymer molecules in the direction of orientation) of
the multilayer optical film 20, because the coPEN of the skin layers
orients very little when stretched under the conditions used to orient the
first optical layers 22.
Preferably, the polyesters of the first optical layers 22, the second
optical layers 24, and the non-optical layers 28 are chosen to have
similar rheological properties (e.g., melt viscosities) so that they can
be co-extruded. Typically, the second optical layers 24 and the
non-optical layers 28 have a glass transition temperature, T.sub.g, that
is either below or no greater than about 40.degree. C. above the glass
transition temperature of the first optical layers 22. Preferably, the
glass transition temperature of the second optical layers 24 and the
non-optical layers 28 is below the glass transition temperature of the
first optical layers 22.
A reflective polarizer may be made by combining a uniaxially-oriented first
optical layer 22 with a second optical layer 24 having an isotropic index
of refraction that is approximately equal to one of the in-plane indices
of the oriented layer. Alternatively, both optical layers 12,14 are formed
from birefringent polymers and are oriented in a multiple draw process so
that the indices of refraction in a single in-plane direction are
approximately equal. The interface between the two optical layers 12,14,
in either case, forms a light reflection plane. Light polarized in a plane
parallel to the direction in which the indices of refraction of the two
layers are approximately equal will be substantially transmitted. Light
polarized in a plane parallel to the direction in which the two layers
have different indices will be at least partially reflected. The
reflectivity can be increased by increasing the number of layers or by
increasing the difference in the indices of refraction between the first
and second layers 22, 24.
Typically, the highest reflectivity for a particular interface occurs at a
wavelength corresponding to twice the combined optical thickness of the
pair of optical layers 22, 24 which form the interface. The optical
thickness of the two layers is n.sub.1 d.sub.1 +n.sub.2 d.sub.2 where
n.sub.1, n.sub.2 are the indices of refraction of the two layers and
d.sub.1, d.sub.2 are the thicknesses of the layers. The layers 22, 24 may
each be a quarter wavelength thick or the layers 22, 24 may have different
optical thicknesses, so long as the sum of the optical thicknesses is half
of a wavelength (or a multiple thereof). A film having a plurality of
layers may include layers with different optical thicknesses to increase
the reflectivity of the film over a range of wavelengths. For example, a
film may include pairs of layers which are individually tuned to achieve
optimal reflection of light having particular wavelengths.
Alternatively, the first optical layers 22 may be biaxially-oriented by
stretching in two different directions. The stretching of optical layers
22 in the two directions may result in a net symmetrical or asymmetrical
stretch in the two chosen orthogonal axes.
One example of the formation of a mirror using a multilayer optical film 20
is the combination of a biaxially-oriented optical layer 22 with a second
optical layer 24 having indices of refraction which differ from both the
in-plane indices of the biaxially-oriented layer. The mirror operates by
reflecting light having either polarization because of the index of
refraction mismatch between the two optical layers 22, 24. Mirrors may
also be made using a combination of uniaxially-oriented layers with
in-plane indices of refraction which differ significantly. In another
embodiment, the first optical layers 22 are not birefringent and a mirror
is formed by combining first and second optical layers 22, 24 which have
significantly different indices of refraction. Reflection occurs without
orientation of the layers. There are other methods and combinations of
layers that are known for producing both mirrors and polarizers and which
may be used. Those particular combinations discussed above are merely
exemplary.
The second optical layers 24 may be prepared with a variety of optical
properties depending, at least in part, on the desired operation of the
multilayer optical film 20. In one embodiment, the second optical layers
24 are made of a polymer material that does not appreciably optically
orient when stretched under conditions which are used to orient the first
optical layers 22. Such layers are particularly useful in the formation of
reflective polarizing films, because they allow the formation of a stack
26 of layers by, for example, coextrusion, which can then be stretched to
orient the first optical layers 22 while the second optical layers 24
remain relatively isotropic. Typically, the index of refraction of the
second optical layers 24 is approximately equal to one of the indices of
the oriented first optical layers 22 to allow transmission of light with a
polarization in a plane parallel to the direction of the matched indices.
Preferably, the two approximately equal indices of refraction differ by
about 0.05 or less, and more preferably by about 0.02 or less, at 632.8
nm. In one embodiment, the index of refraction of the second optical
layers 24 is approximately equal to the index of refraction of the first
optical layers 22 prior to stretching.
In other embodiments, the second optical layers 24 are orientable. In some
cases, the second optical layers 24 have one in-plane index of refraction
that is substantially the same as the corresponding index of refraction of
the first optical layers 22 after orientation of the two sets of layers
22, 24, while the other in-plane index of refraction is substantially
different than that of the first optical layers 22. In other cases,
particularly for mirror applications, both in-plane indices of refraction
of the optical layers 22, 24 are substantially different after
orientation.
A brief description of one method for forming multilayer polymer films is
described. A fuller description of the process conditions and
considerations is found in U.S. patent application Ser. No. 09/006,288
entitled "Process for Making Multilayer Optical Film" incorporated herein
by reference. The multilayer polymer films are formed by extrusion of
polymers to be used in the first and second optical layers, as well as the
non-optical layers. Extrusion conditions are chosen to adequately feed,
melt, mix and pump the polymer resin feed streams in a continuous and
stable manner. Final melt stream temperatures are chosen to be within a
range which reduces freezing, crystallization or unduly high pressure
drops at the low end of the range and which reduces degradation at the
high end of the range. The entire melt stream processing of more than one
polymer, up to and including film casting on a chill roll, is often
referred to as co-extrusion.
Following extrusion, each melt stream is conveyed through a neck tube into
a gear pump used to regulate the continuous and uniform rate of polymer
flow. A static mixing unit may be placed at the end of the neck tube to
carry the polymer melt stream from the gear pump into a multilayer
feedblock with uniform melt stream temperature. The entire melt stream is
typically heated as uniformly as possible to enhance both uniform flow of
the melt stream and reduce degradation during melt processing.
Multilayer feedblocks divide each of two or more polymer melt streams into
many layers, interleave these layers, and combine the many layers into a
single multilayer stream. The layers from any given melt stream are
created by sequentially bleeding off part of the stream from a main flow
channel into side channel tubes which lead to layer slots in the feed
block manifold. The layer flow is often controlled by choices made in
machinery, as well as the shape and physical dimensions of the individual
side channel tubes and layer slots.
The side channel tubes and layer slots of the two or more melt streams are
often interleaved to, for example, form alternating layers. The
feedblock's downstream-side manifold is often shaped to compress and
uniformly spread the layers of the combined multilayer stack transversely.
Thick, non-optical layers, known as protective boundary layers (PBLs), may
be fed near the manifold walls using the melt streams of the optical
multilayer stack, or by a separate melt stream. As described above, these
non-optical layers may be used to protect the thinner optical layers from
the effects of wall stress and possible resulting flow instabilities.
The multilayer stack exiting the feedblock manifold may then enter a final
shaping unit such as a die. Alternatively, the stream may be split,
preferably normal to the layers in the stack, to form two or more
multilayer streams that may be recombined by stacking. The stream may also
be split at an angle other than normal to the layers. A flow channeling
system that splits and stacks the streams is called a multiplier. The
width of the split streams (i.e., the sum of the thicknesses of the
individual layers) can be equal or unequal. The multiplier ratio is
defined as the ratio of the wider to narrower stream widths. Unequal
streams widths (i.e., multiplier ratios greater than unity) can be useful
in creating layer thickness gradients. In the case of unequal stream
widths, the multiplier may spread the narrower stream and/or compress the
wider stream transversely to the thickness and flow directions to ensure
matching layer widths upon stacking.
Prior to multiplication, additional non-optical layers can be added to the
multilayer stack. These non-optical layers may perform as PBLs within the
multiplier. After multiplication and stacking, some of these layers may
form internal boundary layers between optical layers, while others form
skin layers.
After multiplication, the web is directed to the final shaping unit. The
web is then cast onto a chill roll, sometimes also referred to as a
casting wheel or casting drum. This casting is often assisted by
electrostatic pinning, the details of which are well-known in the art of
polymer film manufacture. The web may be cast to a uniform thickness
across the web or a deliberate profiling of the web thickness may be
induced using die lip controls.
The multilayer web is then drawn to produce the final multilayer optical
film. In one exemplary method for making a multilayer optical polarizer, a
single drawing step is used. This process may be performed in a tenter or
a length orienter. Typical tenters draw transversely (TD) to the web path,
although certain tenters are equipped with mechanisms to draw or relax
(shrink) the film dimensionally in the web path or machine direction (MD).
Thus, in this exemplary method, a film is drawn in one in-plane direction.
The second in-plane dimension is either held constant as in a conventional
tenter, or is allowed to neck in to a smaller width as in a length
orienter. Such necking in may be substantial and increase with draw ratio.
In one exemplary method for making a multilayer mirror, a two step drawing
process is used to orient the birefringent material in both in-plane
directions. The draw processes may be any combination of the single step
processes described that allow drawing in two in-plane directions. In
addition, a tenter that allows drawing along MD, e.g. a biaxial tenter
which can draw in two directions sequentially or simultaneously, may be
used. In this latter case, a single biaxial draw process may be used.
In still another method for making a multilayer polarizer, a multiple
drawing process is used that exploits the different behavior of the
various materials to the individual drawing steps to make the different
layers comprising the different materials within a single coextruded
multilayer film possess different degrees and types of orientation
relative to each other. Mirrors can also be formed in this manner.
The following examples demonstrate the manufacture and uses of the
invention. It is to be understood that these examples are merely
illustrative and are in no way to be interpreted as limiting the scope of
the invention.
EXAMPLES
Comparative Example
Formation of an Optical Polarizer with a Polyvinyl Alcohol Dichroic
Polarizing Film. A solution containing 10 wt. % Airvol 107 polyvinyl
alcohol (Air Products, Allentown, Pa.) and 0.1 wt. % Triton X-100 (Union
Carbide, Danbury, Conn.) was coated onto a corona treated unoriented
polyester cast web having four stacks of 209 optical layers, each
utilizing a shoe coater which delivered a wet coating thickness of 64
.mu.m (2.50 mils) of the solution. The coating was dried at 105.degree. C.
for 1 minute. The multilayer polyester cast web and the polyvinyl alcohol
film were oriented simultaneously at 156.degree. C. in a tenter oven in
the direction transverse to the direction of extrusion of the reflective
polarizer. The reflective polarizer and polyvinyl alcohol film were
stretched to 6 times their original width.
The polyvinyl alcohol film was stained in an aqueous iodine/potassium
iodide solution at 35.degree. C. for 20 sec. The staining solution
contained 0.4 wt. % iodine and 21 wt. % potassium iodide. The stain was
fixed in a bath of boric acid/borax at 65.degree. C. for 25 sec. The
fixing solution contained 4.5 wt. % borax and 14.5 wt. % boric acid.
The optical polarizer transmitted 83.5% of light having the desired
polarization and had a Q value of 17. The parameter Q is sometimes
referred to as the dichroic ratio. This ratio, Q, is expressed in terms of
the power transmissions of the high transmission polarization state and
the extinction polarization state as follows:
Q=1n(T.sub.ext)/1n(T.sub.trans)
Where T.sub.trans is the transmission of the high transmission state and
T.sub.ext is the transmission of the extinction state.
The polyvinyl alcohol film showed severe cracking and failed in a cross
hatched tape pull adhesion test. The crosshatched tape pull adhesion test
was performed as follows. First, a sample was placed on a clean, hard
surface. Then, using a plastic template with a 1/8" slot spaced every
1/4", the sample was scribed with a scribing tool to produce a
crosshatched pattern. The scribe went through the coating to the substrate
without going through the substrate. A 4" strip of 1" wide Scotch Brand
#610 (3M Co., St. Paul, Minn.) tape was placed on the diagonal to the
crosshatched pattern. Using the template, the tape was pressed down firmly
to the sample. Next, using one swift movement, the tape was stripped off
the sample at a low angle to the sample surface and in a direction away
from the operator's body. The sample was examined for coating removal. If
no coating has been removed, the sample passes the test. If any coating
has been removed from the sample, the sample failed the test. In this
particular case, the sample failed the test.
Example 1
Formation of an Optical Polarizer with a Polyvinyl Alcohol Dichroic
Polarizing Film Containing a Sulfonated Polyester. An aqueous dispersion
containing 9 wt. % Airvol 107 polyvinyl alcohol (Air Products, Allentown,
Pa.), 1 wt. % WB54 (a sulfonated polyester from 3M Co., St. Paul, Minn.),
3 wt. % N-methylpyrrolidone (available from Aldrich, Milwaukee, Wis.) and
0.1 wt. % Triton X100 (Union Carbide, Danbury, Conn.) was coated onto a
corona treated unoriented multilayer polyester cast web, having four
stacks of 209 optical layers each, using a shoe coater which delivered a
wet coating thickness of 64 .mu.m (2.50 mils) of the polyvinyl alcohol
dispersion. The coating was dried at 105.degree. C. for 1 minute. The
polyvinyl alcohol coating and the multilayer cast web were preheated in a
tenter oven zone heated with hot air charged to 160.degree. C. and then
drawn to six times their original width over 35 seconds in a tenter zone
heated with hot air charged at 150.degree. C. The films were then heated
an additional 85 seconds prior to cooling. The construction exhibited only
a low level of isolated, non-specific defects, possibly due to impurities
or air bubbles.
The polyvinyl alcohol film was stained using an aqueous iodine/potassium
iodide solution at 35.degree. C. for 20 sec. The staining solution
contained 0.4 wt. % iodine and 21 wt. % potassium iodide. The stain was
fixed in a bath of boric acid/borax at 65.degree. C. for 25 sec. The
fixing solution contained 4.5 wt. % borax and 14.5 wt. % boric acid.
The optical polarizer transmitted 87.0% of light having the desired
polarization and had a dichroic ratio, Q, of 17. The substrate passed the
crosshatched tape pull adhesion test.
The present invention should not be considered limited to the particular
examples described above, but rather should be understood to cover all
aspects of the invention as fairly set out in the attached claims. Various
modifications, equivalent processes, as well as numerous structures to
which the present invention may be applicable will be readily apparent to
those of skill in the art to which the present invention is directed upon
review of the present specification. The claims are intended to cover such
modifications and devices.
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